This invention relates to a thermal image recording apparatus with which a recording corresponding to image data is formed on a thermal recording material (hereunder referred to as a "thermal material") using a thermal head. The invention also relates to a recording method for application of that apparatus. More specifically, the invention relates to a thermal image recording apparatus and method which are capable of forming recording at high speed without uneven densities.
Thermal materials comprising a thermal recording layer on a substrate such as a paper or film are commonly used to record the images produced in diagnosis by ultrasonic scanning. This recording method, commonly referred to as thermal image recording, eliminates the need for wet processing and offers several advantages including convenience in handling. Hence, the use of the thermal image recording system is not limited to small-scale applications such as diagnosis by ultrasonic scanning and an extension to those areas of medical diagnoses such as CT, MRI and X-ray photography where large and high-quality images are required, is under review.
As is well known, the thermal image recording apparatus uses a thermal head having a glaze in which heat generating resistors corresponding to the number of pixels of one line are arranged in one direction and, with the glaze slightly pressed against the thermal recording layer of the thermal material, the two members are moved relative to each other in a direction approximately perpendicular to the direction in which the heat generating resistors are arranged, and the respective heat generating resistors of the glaze are heated in accordance with the image to be recorded to heat the thermal recording layer imagewise, thereby accomplishing image reproduction.
A typical method of heating the individual heat generating resistors is by applying an electric current to such resistors for specified time periods that correspond to the image data of the individual pixels in the image to be recorded. However, the temperatures of the heat generating resistors to be energized vary from each other depending on the history of heat generation up to the previous line and, therefore, even if the heat generating resistors corresponding to the pixels having the same image data in the present line are energized for the same time period, temperature differences will occur between the heated resistors, thereby producing unevenness in the recording density.
In order to solve this problem of uneven recording densities, the image data must be compensated for temperature such that the heat generating temperature for the image data are corrected for each heat generating resistor on the basis of that image data and the history of heat generation up to the previous line.
Unexamined Published Japanese Patent Application 59-98878 teaches a thermal recording apparatus capable of outputting images at consistent density during high-speed recording. This apparatus performs thermal transfer recording using an ink ribbon and comprises memory means for storing the quantity of energy stored in each of the heat generating resistors, first computing means by which the electric energy to be applied to each of the heat generating resistors is calculated on the basis of the output data from said memory means and the input image data, second computing means by which the electric energy stored in each of the heat generating resistors is calculated on the basis of the output data from the memory means and the input image data, and control means by which the quantity of the electric energy to be applied to each of the heat generating resistors is controlled in accordance with the output of the first computing means.
In the above thermal recording apparatus, the electric energy to be applied at the present time is calculated on the basis of the image data to each of the heat generating resistors and the quantity of the heat stored up to the present time, namely, the past image data weighted to have a progressively smaller value back into the past; therefore, according to the patent, the calculated results reflect the changes in the temperatures of the individual heat generating resistors more correctly and, compared to the conventional system, the apparatus can provide more uniform recording densities, which is an advantage particularly salient in a high-speed recording mode.
In fact, however, the apparatus is designed to be such that the electric energy to be applied to the heat generating resistor corresponding to each one of the pixels is calculated for the entire surface of one screen, so that quite a lot of time is required to calculate the electric energy of interest. Therefore, if the size of one screen increases or if the number of recording pixels is increased in order to meet the demand for producing images of higher quality, it becomes difficult to achieve high-speed recording with this apparatus. If a capability for high-speed calculation is needed, the system configuration must be made complex enough which increases the manufacturing cost.